The subject invention relates to the field of battery fuel gauging. The subject method and apparatus provides for an accurate recalibration of a battery""s state of charge (SOC) and/or an accurate estimation of a battery""s SOC. The subject invention is particularly advantageous for application to batteries which operate such that full discharge and/or full charge of the battery occurs infrequently. It is often important to have an accurate SOC determination so that a reliable prediction can be made as to how much stored energy a battery has remaining and/or to optimize the charging and discharging of the battery.
Traditional battery fuel gauging methods and apparatus utilize hardware and algorithms to detect full charge and/or full discharge and then perform coulometric measurements to estimate the change in the SOC with time. These coulometric measurements typically utilize correction factors to account for battery non-linearities such as charge efficiency, self-discharge, and deliverable capacity. The error in the SOC estimate based on these coulometric measurements will continue to accumulate until a recalibration can occur. The calibration process can involve processing measurable battery parameters that have been empirically, or theoretically, shown to provide a good correlation to a specific SOC. Full charge and/or full discharge are typically considered known SOC points and can then be used to recalibrate the fuel gauge measure and reset the estimated error.
Full charge determinations are dependent upon cell chemistry and are well known in the industry. Full discharge has generally been correlated to an end-of-discharge-voltage (EODV). These calibration methods have been shown to work well in applications such as power tools, computers, cellular phones, etc., where the batteries are allowed to cycle through these known calibration points frequently during normal usage.
However, with respect to some battery applications, the frequency of achieving full charge and/or full discharge is low. For example, in some hybrid electric vehicle (HEV) applications it is preferable that the battery SOC stay within an operating range around a SOC set point. This set point is sometimes referred to as the xe2x80x9csweet spotxe2x80x9d. As an example, the sweet spot can be 50% SOC with battery SOC preferably confined to an operating range of +/xe2x88x9230%. Accordingly, in this situation, the battery will remain between 20% SOC and 80% SOC and, therefore, will not typically reach the traditional calibration points of a fully charged state and/or a fully discharged state. Accordingly, the error in a SOC estimate based on coulometric measurements will tend to grow with time of use. This growing error can result in undesirable vehicle performance. If the battery SOC estimate is too high, the vehicle can be left with minimal power for peak loads such as climbing hills, acceleration, and restarting the vehicle. If the battery SOC estimate is too low, the vehicle system efficiency can be reduced because of the battery""s reduced charge efficiency at high SOC.
Accordingly, there is a need for a method and apparatus for allowing recalibration of a battery""s SOC in situations where the battery infrequently, if ever, reaches a fully charged state and/or a fully discharged state. In particular, HEV battery applications which operate within a range centered at a SOC sweet spot need a means of recalibrating the SOC to maintain a more accurate SOC estimate. There is also a need for a method and apparatus for allowing determination of a battery""s SOC during the operation of the battery.
The subject invention pertains to a method and apparatus for estimating a state of charge (SOC) of a battery. A specific embodiment of the subject method comprises generating a curve which expresses the relationship between a parameter and a state of charge of a battery over a region of interest; measuring a voltage and a current of the battery during a period of time to create a set of voltage and current data; processing the set of voltage and current data to create a set of processed data; regressing the set of processed data with respect to an equation which is representative of the battery to obtain a value of the parameter; and comparing the value of the parameter to the curve in order to obtain an estimate of the state of charge of the battery.
The subject method and apparatus can be used to recalibrate a SOC of a battery. The subject invention is advantageous in situations where the battery infrequently, if ever, reaches a fully charged state and/or a fully discharged state. In one embodiment, the subject method and apparatus can utilize a battery""s voltage and/or current, and/or changes in a battery""s voltage and/or current with time, to recalibrate the battery""s SOC and/or estimate the battery""s SOC.
Referring to FIG. 1, the open-circuit-voltage (OCV) for many battery chemistries changes monatonically with SOC. Additionally, the OCV versus SOC curve typically has an inflection point. The method and apparatus of the subject invention can monitor various parameters relating to the condition of the battery in order to determine where on the OCV versus SOC curve the battery is and, in a specific embodiment, determine when the battery reaches the inflection point. In a specific embodiment, the method and apparatus of the subject invention can monitor the second derivative of OCV versus SOC and compare with the battery""s OCV versus SOC curve to estimate the SOC. The battery""s OCV versus SOC curve used for comparison can be determined, for example, prior to use of the battery or during use of the battery. Accordingly, the subject invention can be used to determine when the battery is outside a certain SOC region, such as the 30% SOC to 70% SOC region or the 20% SOC to 80% SOC region. If desired, the variation of the OCV versus SOC curve with respect to, for example, temperature, battery age, charge rate, and/or discharge rate can be taken into account during the recalibration of and/or estimation of SOC in order to enhance the accuracy of such SOC recalibration and/or estimation.